Device and method of calibrating fiber Bragg grating based fiber optic overheat systems

ABSTRACT

A system configured to monitor a plurality of zones of an aircraft includes a line replaceable unit, a first interrogator, and a controller. The line replaceable unit includes first and second connectors in optical communication and an optical fiber. The optical fibers includes a first plurality of fiber Bragg gratings and a plurality of calibration fiber Bragg gratings in a pattern providing information related to a calibration value based upon a center wavelength of each of the first plurality of fiber Bragg gratings. The first interrogator is connected to the line replaceable unit at the first end of the optical fiber and is configured to provide a first optical signal and to receive a first optical response signal from the optical fiber. The controller is operatively connected to the first interrogator and is configured to determine the calibration value of the line replaceable unit.

BACKGROUND

This disclosure relates generally to aircraft system health monitoringfor overheat and fire detection systems. More particularly, thisdisclosure relates to aircraft system health monitoring using opticalsignals.

Prior art overheat detection systems typically utilize eutectic salttechnology to sense an overheat event. The eutectic salt surrounds acentral conductor and the eutectic salt is surrounded by an outersheath. A monitoring signal is transmitted along the central conductor,and under normal operating conditions the eutectic salt operates as aninsulator such that no conduction occurs between the central conductorand the outer sheath. When an overheat event occurs, however, a portionof the eutectic salt melts and a low-impedance path is formed betweenthe central conductor and the outer sheath. The low-impedance path issensed by an electronic controller, which generates an overheat alarmsignal. When the overheat event has subsided, the eutectic saltre-solidifies and once again insulates the central conductor. Throughthe use of various salts to create a eutectic mixture, a specificmelting point for the salt can be achieved. Accordingly, differenteutectic salts can be used in different areas of the aircraft to provideoverheat monitoring across a variety of temperatures. While the eutecticsalt technology enables detection of overheat events, the eutectic salttechnology merely provides a binary indication of whether an overheatevent has or has not occurred.

SUMMARY

A system configured to monitor a plurality of zones of an aircraftincludes a line replaceable unit, a first interrogator, and acontroller. The line replaceable unit includes first and secondconnectors in optical communication and an optical fiber extendingbetween the first and second connectors. The first end of the opticalfiber is connected to the first connector. The optical fibers includes afirst plurality of fiber Bragg gratings disposed in the optical fiberand a plurality of calibration fiber Bragg gratings located in a patternthat provides information related to a calibration value of the linereplaceable unit based upon a center wavelength of each of the firstplurality of fiber Bragg gratings. The first interrogator is connectedto the line replaceable unit at the first end of the optical fiber andis configured to provide a first optical signal to the optical fiber andto receive a first optical response signal from the optical fiber. Thecontroller is operatively connected to the first interrogator and isconfigured to determine the calibration value of the line replaceableunit.

A method of calibrating a fiber optic overheat system includes emittinga first optical signal into the optical fiber with a first opticaltransmitter disposed in a first interrogator connected to an opticalfiber. The optical fiber includes a plurality of overheat fiber Bragggratings disposed in the optical fiber, and a plurality of calibrationfiber Bragg gratings disposed in the optical fiber. The first opticalsignal is reflected with at least one of the plurality of calibrationfiber Bragg gratings to create a response signal. The response signalfrom the optical fiber based upon the reflected first optical signal isreceived by a first optical receiver in the first interrogator. Thereceived response signal is detected to identify presences of each ofthe plurality of calibration fiber Bragg gratings. A calibration valueis determined based upon the identified presences of the plurality ofcalibration fiber Bragg gratings.

A detection system includes a line replaceable unit, a firstinterrogator, a second interrogator, and a controller. The linereplaceable unit includes first and second connectors in opticalcommunication, and an optical fiber extending between the first andsecond connectors. A first end of the optical fiber is connected to thefirst connector. The optical fiber includes a plurality of overheatfiber Bragg gratings, a first timing fiber Bragg grating, and aplurality of calibration fiber Bragg gratings. The first timing fiberBragg grating is configured to indicate at least one of a start pointand end point of the line replaceable unit. The plurality of calibrationfiber Bragg gratings are located in a pattern that provides informationrelated to a calibration value of the line replaceable unit based upon acenter wavelength of each of the first plurality of overheat fiber Bragggratings. The first interrogator is connected to the line replaceableunit at the first end of the optical fiber and is configured to providea first optical signal to the optical fiber and to receive a firstoptical response signal from the optical fiber. The second interrogatoris connected to the second end of the optical fiber and is configured toprovide a second optical signal to the optical fiber and to receive asecond optical response signal from the optical fiber. The controller isoperatively connected to the first interrogator and is configured todetermine the calibration value of the line replaceable unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an overheat detection system architecturefor monitoring multiple zones.

FIG. 2 is a flow diagram illustrating example operations to provideoverheat detection in an aircraft utilizing optical signals.

FIG. 3 is a flow diagram illustrating example operations using opticalsignals to provide health monitoring for an aircraft.

FIG. 4A is a simplified block diagram of a fiber optic event detectionsystem with a single line replaceable unit including overheat fiberBragg gratings and temperature fiber Bragg gratings.

FIG. 4B is a simplified block diagram of a fiber optic event detectionsystem with two line replaceable units including overheat fiber Bragggratings and temperature fiber Bragg gratings.

FIG. 5A is a block diagram of a multi-channel interrogator with opticalswitches positioned downstream of couplers.

FIG. 5B is a block diagram of a multi-channel interrogator with opticalswitches positioned upstream of couplers.

FIG. 6 is a block diagram of a multi-channel interrogator with a 1×Noptical switch.

FIG. 7 is a simplified block diagram of a fiber optic event detectionsystem with a single line replaceable unit including overheat fiberBragg gratings, temperature fiber Bragg gratings, and timing markerfiber Bragg gratings.

FIG. 8 is a graph depicting a response signal from the overheatdetection system and a series of sample points.

FIG. 9A is a simplified block diagram of a fiber optic event detectionsystem with a single line replaceable unit including overheat fiberBragg gratings, temperature fiber Bragg gratings, timing marker fiberBragg gratings, and calibration fiber Bragg gratings disposed in a firstpattern.

FIG. 9B is a simplified block diagram of a fiber optic event detectionsystem with a single line replaceable unit including overheat fiberBragg gratings, temperature fiber Bragg gratings, timing marker fiberBragg gratings, and calibration fiber Bragg gratings disposed in asecond pattern.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of overheat detection system 10 foraircraft 12. Aircraft 12 includes zones Za-Zj and avionics controller14. Overheat detection system 10 includes interrogators 16 a-16 b andoptical fibers 18 a-18 c. Interrogator 16 a includes optical transmitter20 a, detector 22 a, and computer-readable memory 24 a. Interrogator 16b includes optical transmitter 20 b, detector 22 b, andcomputer-readable memory 24 b. Optical fibers 18 a-18 c include firstends 28 a-28 c and second ends 30 a-30 c.

Overheat detection system 10 is a system for detecting overheat eventsand/or specific temperature values throughout various areas of aircraft12. Aircraft 12 is an airplane, helicopter, or other machine capable offlight. Zones Za-Zj may include any one or more locations on aircraft 12where overheat detection is desired. For example, zones Za-Zj mayinclude bleed air ducts, cross-over bleed air ducts, wheel wells, wingboxes, air conditioning (A/C) packs, anti-icing systems, nitrogengeneration systems, or any other area where temperature sensing isdesirable. While aircraft 12 is described as including ten zones, itshould be understood that aircraft 12 may be divided into as many or asfew zones as desired. Aircraft 12 may be divided into zones in anydesired manner; for example, aircraft 12 may be divided into zones basedon the overheat temperature for the components located in that zone orbased on system type. Each zone Za-Zj of aircraft 12 may have adifferent alarm set point. For instance, when the temperature in zone Zais the same as the temperature in zone Zb, an overheat alarm may betriggered for zone Zb but not for zone Za.

Avionics controller 14 is a digital computer and can include one or moreelectronic control devices. In one non-limiting embodiment, avionicscontroller 14 can be a part of first or second interrogators 16 a or 16b. In another non-limiting embodiment, avionics controller 14 can beomitted from overheat detection system 10 and such that first and orsecond interrogators 16 a and 16 b will determine all information,including zone configuration, the number of zones, temperaturethreshold, overheat detection, and other functionality of an avionicscontroller. In such a non-limiting embodiment, first and secondinterrogators 16 a and 16 b are connected with a communication channelso as to communicate with each other. Each of interrogators 16 a and 16b may be a microprocessor, a microcontroller, application-specificintegrated circuit (ASIC), a digital signal processor (DSP), a fieldprogrammable gate-array (FPGA), or other equivalent discrete orintegrated logic circuitry. In this and other non-limiting embodimentsdiscussed herein, interrogators 16 a and 16 b are fiber Bragg grating(FBG) interrogators (see e.g., FIGS. 2-9B). Interrogators 16 a and 16 bare substantially similar, and for ease of discussion, interrogator 16 awith optical transmitter 20 a, detector 22 a, and computer-readablememory 24 a will be discussed in further detail.

Optical fibers 18 a, 18 b, and 18 c are fiber optic cables configured tocommunicate an optical signal. Optical fibers 18 a, 18 b, and 18 c aresubstantially similar, and for ease of discussion, optical fibers 18 awith first end 28 a and second end 30 a will be discussed in furtherdetail. Optical fiber 18 a is illustrated as including first end 28 aand second end 30 a. It should be understood that while optical fiber 18a is illustrated as including a single fiber optic cable, each ofoptical fibers 18 a-18 c can include one or more fiber optic cables. Inother non-limiting embodiments, optical fibers 18 a-18 c can include oneor more line replaceable units (LRUs) that divide optical fibers 18 a-18c into separate, but connectable optical fiber segments. Throughout thisdisclosure, the term channel is synonymous with the optical fiber, andas such the two terms can be used interchangeably to refer to the samerespective element.

Optical transmitter 20 a may be any suitable optical source forproviding an optical signal. In one non-limiting embodiment, opticaltransmitter 20 a may be a light-emitting diode or a laser. It should befurther understood that optical transmitter 20 a may be configured toprovide the optical signal in any suitable manner, such as through asingle pulse at a fixed wavelength, a tunable swept-wavelength, abroadband signal, and/or a tunable pulse. Detector 22 a is a receiverconfigured to receive an optical signal. For example, detector 22 a maybe a photodiode, a photodiode array, a phototransistor, a circulator, orany other suitable optical receiving device. While interrogator 16 a isdescribed as including a single detector 22 a, it should be understoodthat interrogator 16 a may include multiple optical receivers to receivethe optical signal from different optical fibers, different fiber opticcables, and/or different ends of the fiber optic cables.

Computer-readable memory 24 a can be configured to store electronicinformation during and after operation of aircraft 12. In onenon-limiting embodiment, computer-readable memory 24 a can be describedas a computer-readable storage medium. In one non-limiting embodiment, acomputer-readable storage medium can include a non-transitory medium.The term “non-transitory” can indicate that the storage medium is notembodied in a carrier wave or a propagated signal. In one non-limitingembodiment, a non-transitory storage medium can store data that can,over time, change (e.g., in RAM or cache). In one non-limitingembodiment, computer-readable memory 24 a can include temporary memory,meaning that a primary purpose of the computer-readable memory is notlong-term storage. In one non-limiting embodiment, computer-readablememory 24 a can be described as a volatile memory, meaning that thecomputer-readable memory 24 a does not maintain stored contents whenelectrical power is removed. In one non-limiting embodiment, examples ofvolatile memories can include random access memories (RAM), dynamicrandom access memories (DRAM), static random access memories (SRAM), andother forms of volatile memories. Couplers 26 a and 26 b are opticaldevices with one or more optical inputs and one or more optical outputs,and which are capable of splitting an optical signal into multiplechannels. First end 28 a and second end 30 a are opposite ends ofoptical fiber 18 a.

Overheat detection system 10 is disposed within and throughout variouszones Za-Zj of aircraft 12. In this non-limiting embodiment, opticalfiber 18 a passes through zones Zb-Zd, optical fiber 18 ab passesthrough zones Za and Ze-Zg, and optical fiber 18 ac passes through zonesZh-Zj. As such, each optical fiber 18 a-18 c passes through and gathersinformation regarding multiple zones of aircraft 12. Avionics controller14 is mounted within aircraft 12 and is electrically connected tointerrogators 16 a and 16 b. Interrogator 16 a is connected to avionicscontroller 14 to communicate information to avionics controller 14.Interrogator 16 a is connected to optical transmitter 20 a to controlthe transmission of an optical signal from optical transmitter 20 a tofiber optic cable 18 a. Interrogator 16 a is also connected to detector22 a to analyze the signals received by detector 22 a.

Optical fibers 18 a-18 c are substantially similar, and for purposes ofclarity and ease of discussion, optical fiber 18 a will be discussed infurther detail. Optical fiber 18 a passes through each of zones Zb-Zdand is connected to interrogator 16 a and interrogator 16 b. Opticalfiber 18 a is in optical communication with detector 22 a ofinterrogator 16 a and with detector 22 b of interrogator 16 b. Opticalfiber 18 a is connected to interrogator 16 a on first end 28 a and tointerrogator 16 b on second end 30 a. Optical fiber 18 b is connected tointerrogator 16 a on first end 28 b and to interrogator 16 b on secondend 30 b. Optical fiber 18 c is connected to interrogator 16 a on firstend 28 c and to interrogator 16 b on second end 30 c. Interrogators 16 aand 16 b are connected to avionics controller 14 to communicate withother systems within aircraft 12.

Optical transmitter 20 a is mounted within interrogator 16 a and is inoptical communication with optical fiber 18 a via coupler 26 a. Detector22 a is mounted within interrogator 16 a and is in optical communicationwith optical fiber 18 a via coupler 26 a. Computer-readable memory 24 ais mounted within interrogator 16 a and is communication with opticaltransmitter 20 a and detector 22 a. Coupler 26 a is mounted withininterrogator 16 a and is in optical communication with opticaltransmitter 20 a, detector 22 a, and optical fiber 18 a. First end 28 aof optical fiber 18 a is connected to interrogator 16 a and is inoptical communication with coupler 26 a and with second end 30 a ofoptical fiber 18 a. Second end 30 a of optical fiber 18 a is connectedto interrogator 16 b and is in optical communication with coupler 26 band with first end 28 a of optical fiber 18 a.

Overheat detection system 10 can sense a temperature or strain at anylocation or at multiple locations along optical fiber 18 a. Because thetemperature can be sensed at any location or multiple locations alongoptical fiber 18 a, a temperature profile may be developed for theentire lengths of optical fiber 18 a, 18 b, and 18 c, and as such, atemperature profile may be developed for each zone Za-Zj. Overheatdetection system 10 can further provide locational information regardinga determined location within each zone Za-Zj at which an event occurs.The temperature profile for each zone Za-Zj can then be compared to amaximum allowable temperature profile, which can include a singletemperature for an entire zone Za-Zj or multiple temperatures at varyinglocations in each zone Za-Zj. It should be understood thatcommunications for overheat detection system 10 can be made using anycombination of wired, wireless, or optical communications.

Aircraft 12 may include a central overheat detection system computerthat communicates with various overheat detection systems on aircraft12, and the central overheat detection system computer may communicateany overheat status from any overheat detection system to the cockpit.Avionics controller 14 communicates information from interrogators 16 aand 16 b to other systems within aircraft 12.

Interrogators 16 a-16 b can communicate with avionics controller 14, andavionics controller 14 can consolidate the information received frominterrogators 16 a-16 b and provide the information to the cockpit,provide the information to maintenance personnel, and/or store theinformation to generate trend data. While interrogators 16 a-16 b aredescribed as communicating with avionics controller 14, it should beunderstood that interrogators 16 a-16 b can communicate directly withthe cockpit or ground personnel, can store the information to generatetrend data, and/or can communicate with a central overheat computer. Itshould be understood that all communications for overheat detectionsystem 10 can be made using wired, wireless, or optical communicationsor some combination of these methods.

While interrogator 16 a is described as communicating with avionicscontroller 14, interrogator 16 a may communicate with aircraft 12 andwith maintenance personnel in any suitable manner. Interrogator 16 a mayalso communicate directly with a cockpit of aircraft 12 to provideoverheat or fire detection warning, or to indicate that maintenance isnecessary. Interrogator 16 a may further communicate temperature data toother system computers, which may communicate an overheat status to thecockpit. Interrogator 16 a may further communicate with avionicscontroller 14 to communicate temperature data to avionics controller 14using a wired or wireless connection.

Interrogator 16 a may be configured to control optical transmitter 20 ato control the transmission of an optical signal through optical fiber18 a. Interrogator 16 a may also be configured to receive an opticalsignal from detector 22 a and to analyze the optical signal received atdetector 22 a. Interrogator 16 a receives information regarding theoptical signal from detector 22 a. Variations in the optical signalsanalyzed by interrogator 16 a allow interrogator 16 a to determine thetemperature within zones Za-Zj and to determine a location oftemperature variation within zones Za-Zj. The variations in the opticalsignals also allow interrogator 16 a to determine the strain experiencedat various locations along optical fiber 18 a. Interrogator 16 a isconfigured to determine the occurrence of an overheat event, the zone inwhich the overheat event has occurred in, and whether the overheat eventis at or above the alarm set point for that zone. Interrogator 16 atherefore identifies the length and alarm set point of optical fiber 18a in each zone Za-Zj and the order in which optical fiber 18 a passesthrough each zone Za-Zj.

Interrogator 16 a can also generate trend data to facilitate healthmonitoring of aircraft 12. The trend data may include data regardingtemperature trends, strain trends, or both. The trend data can be storedin memory 24 a of interrogator 16 a or in any other suitable storagemedium at any other suitable location, such as the memory of avionicscontroller 14. It should be understood that the data can be monitored inreal time. In one non-limiting embodiment, interrogator 16 a maycommunicate with a dedicated health monitoring system to monitor thetemperature data in real time. The stored trend data providesstatistical and historical data for the temperature, strain (or both)experienced in all zones Za-Zj. The temperature trend data may be storedand monitored by maintenance personnel. As such, the temperature trenddata allows maintenance personnel to determine the location ofprogressive temperature increases over time.

It should be further understood that interrogator 16 a can generate thelocation of a one-time temperature variation, strain variation, or both.Generating the locations of progressive temperature increases allows forpreventative, targeted maintenance before a failure occurs. For example,the temperature trend in a right wheel well may be monitored to generatetrend data. The trend data may show that a tire within the right wheelwell exceeds the normal operating temperatures without reaching thealarm set point. In such a case, an overheat event does not occur;however, the temperature trend data informs maintenance personal thatthe tire may be close to failing or that the tire may be low on airpressure and that a maintenance action is required. Similar totemperature monitoring, the strain trend data may be stored and areas ofincreased strain may be located. In one non-limiting embodiment, thepressure of the bleed air passing through a bleed duct may impart astrain on the wall of the bleed duct. The level of the strain and thelocation of the strain may be detected by interrogator 16 a analyzingthe information received from the optical signals. The straininformation may then be communicated to ground personnel and used toinvestigate the location of the increased strain to determine anymaintenance action that should be taken.

Optical fibers 18 a, 18 b, and 18 c are configured to transmit and/orcommunicate an optical signal. As will be discussed with reference toother figures, FBG sensors disposed along optical fibers 18 a, 18 b, and18 c are used to determine linear expansion of optical fibers 18 a, 18b, and 18 c throughout operation of aircraft 12. As such, optical fibers18 a, 18 b, and 18 c can provide temperature and/or strain sensingacross all zones Za-Zj. Optical transmitter 20 a provides an opticalsignal to optical fibers 18 a, 18 b, and 18 c. Optical transmitter 20 ais configured to provide an optical signal to first end 28 a of opticalfiber 18 a. It should be understood that a single optical transmitter 20a may provide the same optical signal to each of optical fibers 18 a, 18b, and 18 c.

Detector 22 a is configured to receive either optical reflection signalsexcited by optical transmitter 20 a or optical transmission signalsexcited by optical transmitter 20 b. Where optical transmitter 20 aprovides the optical signal through first end 28 a, the optical signaltravels through optical fiber 18 a and is reflected back to first end 28a and received by detector 22 a. Detector 22 a communicates informationregarding the first portion of the optical signal, the second portion ofthe optical signal, or both to interrogator 16 a. In some non-limitingexamples, computer-readable memory 24 a can be used to store programinstructions for execution by one or more processors of interrogator 16.For instance, computer-readable memory 24 a can be used by software orapplications executed to temporarily store information during programexecution.

Coupler 26 a splits an optical signal received from optical transmitter20 a into optical signals for each of optical fibers 18 a, 18 b, and 18c. In this non-limiting embodiment, coupler 26 a includes a 2×3configuration (e.g., 2 inputs and 3 outputs). In other non-limitingembodiments, coupler 26 a can include one or more couplers including N×Mconfigurations, wherein N and M can be any number of inputs and outputs.First end 28 a is configured to communicate an optical signal frominterrogator 16 a to optical fiber 18 a and to communicate an opticalsignal from optical fiber 18 a to interrogator 16 a. Second end 30 a isconfigured to communicate an optical signal from optical fiber 18 a tointerrogator 16 b and to communicate an optical signal from interrogator16 b to optical fiber 18 a.

Different systems within aircraft 12 require overheat detectionmonitoring, and each system may be divided into multiple zones. Forexample, a bleed air duct in aircraft 12 may include multiple zones witha single optical fiber extending through all of the zones of the bleedair duct. Each system may thus be divided into multiple zones and mayinclude a dedicated interrogator and optical fiber. It should beunderstood, however, that aircraft 12 may be divided into zones in anydesired manner.

First end 28 a of optical fiber 18 a receives an optical signal fromoptical transmitter 20 a located within interrogator 16 a, optical fiber18 a transmits the optical signal through optical fiber 18 a to secondend 30 a, and second end 30 a transmits the optical signal to detector22 b located within interrogator 16 b. Interrogator 16 b analyzes thesignal received by detector 22 a to determine the temperature in zonesZb-Zd. Each zone Zb-Zd may have a different alarm set point as thetemperature resistance of each zone may differ. As such, interrogator 16b analyzes the information received to determine the temperature in eachzone. In addition to determining temperature in zones Zb-Zd,interrogator 16 b can analyze the information received from opticalfiber 18 a to determine the strain experienced in each zone Zb-Zd.Interrogator 16 b can thus monitor temperature, strain, or both withinzones Zb-Zd. While optical fiber 18 a is described as being connected tointerrogators 16 a and 16 b, it should be understood that optical fiber18 a can be disposed in a single-ended configuration such that only oneof first end 28 a and second end 30 a is connected to interrogator 16 a.For example, in the single-ended configuration where first end 28 a isconnected to interrogator 16 a, interrogator 16 a can provide an opticalsignal to first end 28 a of optical fiber 18 a and can interpret thesignal that is reflected back through first end 28 a.

Additional examples of fiber optic overheat detection systems can befound in co-pending U.S. patent application Ser. No. 15/600,100 filed onMay 19, 2017, which is herein incorporated by reference in its entirety.With continued reference to FIG. 1, FIGS. 2-3 are flow diagramsillustrating example operations for determining the occurrence andlocation of an overheat event. For purposes of clarity and ease ofdiscussion, the example operations are described below within thecontext of overheat detection system 10. The non-limiting embodimentsdiscussed herein can be for any FBG sensing system regardless of what isbeing measured (i.e., temperature, or otherwise).

FIG. 2 is a flow diagram illustrating example operations to provideoverheat detection in an aircraft utilizing optical signals. In step 32,an optical signal is provided to one or more fiber optic cables, such asoptical fibers 18 a-18 c. For example, optical transmitter 20 a canprovide an optical signal to optical fiber 18 a through first end 28. Instep 34, an optical response signal is received by detector 22 a fromoptical fiber 18 a. For instance, detector 22 a may receive the opticalresponse signal from optical fiber 18 a, and detector 22 a may providethe optical response signal to interrogator 16 a. In step 36, theoptical response signal is analyzed to determine the temperature,strain, or both along optical fiber 18 a. For example, interrogator 16 amay analyze the optical response signal received from detector 22 a todetermine the actual temperature and/or strain at various locationsalong optical fiber 18 a. Interrogator 16 a may use any suitable methodto analyze the optical response, such as the methods discussed below. Itshould be understood that optical fiber 18 a may sense a temperature atany location along optical fiber 18 a and the optical signal can beinterrogated to determine the precise location at which the temperaturechange occurs. As such, the temperature data analyzed by interrogator 16a may include information to determine a temperature at a singlelocation within a zone, a temperature at multiple locations throughout azone, a temperature profile for a zone, or any other temperatureinformation for the zone. In step 38, the temperature data and/or straindata generated in step 36 is compared against a threshold. Where thetemperature data and/or strain data indicates that the temperatureand/or strain are below the threshold level, the operation returns tostep 32. Where the temperature data and/or strain data indicates thatthe temperature and/or strain are above the threshold level, theoperation proceeds to step 40 and the existence of the overheatcondition is indicated and communicated to the cockpit and/or groundpersonnel.

FIG. 3 is a flow diagram illustrating example operations using opticalsignals to provide health monitoring for an aircraft. In step 42, anoptical signal is provided to one or more fiber optic cables, such asoptical fibers 18 a-18 c. In step 44, an optical response signal isreceived from optical fiber 18 a. In step 46, the optical responsesignal is analyzed to determine the temperature, strain, or bothexperienced along optical fiber 18 a. In step 48, the temperature data,strain data, or both is stored in a memory. For example, temperaturedata may be stored in memory 24 a of interrogator 16 a. In step 50,trends are developed for the stored temperature data and/or strain data,and the trends are monitored for any patterns indicating that amaintenance action is necessary.

By utilizing optical fiber 18 a to determine the existence of anoverheat event, prior art eutectic salt sensors, and therefore theelectrical connections associated with the eutectic salt sensors, may beeliminated from aircraft 12. The prior art eutectic salt sensors sensewhether an overheat event is or is not occurring, and as such provide abinary response. Unlike the prior art eutectic sensors, optical fiber 18a senses any changes in temperature and the location of the temperaturechange, not merely whether a temperature set point has been exceeded. Assuch, interrogator 16 a may gather trend data for each zone that opticalfiber 18 a extends through, as data is continuously gathered byinterrogator 16 a. Temperature trend data provides information tomaintenance personnel regarding the overall health of each zone Za-Zj.Providing the trend data allows for maintenance to be performed atspecific, relevant locations and only when needed, thereby decreasingthe downtime of aircraft 12. In addition to providing temperature trenddata, optical fiber 18 a is able to sense strain within each zone Za-Zj,unlike the prior art eutectic salt sensors that are sensitive totemperature alone. Utilizing optical fiber 18 a thus provides additionalstructural information to maintenance personnel.

Monitoring the temperature trend, strain trend, or both within zonesZa-Zj provides information regarding the overall health of the zonebeing monitored, and of the system within which the zone is located. Thetrend data can be used to facilitate preventative maintenance. Moreover,monitoring the trend data allows for maintenance actions to be scheduledat a convenient time and location, instead of waiting until an actualfailure occurs, which can lead to gate departure delay, cancelledflights, or in-flight crew action. In addition, monitoring the actualtemperature in zones Za-Zj enables overheat detection system 10 toprovide fire monitoring in addition to overheat detection. A sudden,dramatic increase in temperature can indicate the existence of a fireinstead of an overheat event. For example, a fire in a wheel well wouldcause a sudden, dramatic increase in temperature in the wheel well, andthat sudden, dramatic increase would be sensed by the portion of thefiber optic cable passing through the zone that includes the wheel well.Interrogator 16 a can analyze the data provided from the zone thatincludes the wheel well to determine the existence of the fire event,and to communicate the existence of the fire event to the cockpit, to afire suppression system, or to any other appropriate system orpersonnel.

A variety of fiber optic cables and operating principles may be used todetermine the existence of an overheat event. For example, overheatdetection system 10 may utilize a single fiber optic cable, dual fiberoptic cables, and fiber optic cables including FBGs. Moreover, the fiberoptic cables may be arranged in a single loop configuration, a dual loopconfiguration, or any other suitable configuration. An optical signal isinitially provided to optical fiber 18 a, and as the optical signaltravels through optical fiber 18 a the majority of the optical signaltravels from first end 28 a to second end 30 a, but a fraction of theoptical signal is backscattered towards first end 28 a. Interrogators 16a and 16 b can analyze the portion of the optical signal receivedthrough second end 30, the portion of the optical signal backscatteredthrough first end 28 a, or a combination of both to determinetemperature and/or strain information. As such, it should be furtherunderstood that optical fiber 18 a can be arranged in a single-endedconfiguration where one of first end 28 a or second end 30 a isconnected to one of interrogator 16 a or interrogator 16 b. In asingle-ended configuration, interrogator 16 a can provide the opticalsignal through one end of optical fiber 18 a and can interpret theportion of the optical signal backscattered through the end of opticalfiber 18 a connected to interrogator 16 b.

Where optical fiber 18 a includes FBGs, interrogator 16 a can analyzethe optical signal using a variety of principles, including WaveDivision Multiplexing (WDM), Time Division Multiplexing (TDM), and/or acombination of WDM and TDM (WDM/TDM), among others. A FBG is adistributed reflector within the fiber optic cable that is configured toreflect a particular wavelength of light and allow all other wavelengthsto pass through. As such, the FBGs function as wavelength-specificreflectors. The specific wavelength reflected by a specific FBG is theBragg wavelength. In overheat detection system 10, optical fiber 18 aincludes various FBGs within optical fiber 18 a. Different FBGs may bedisposed within different zones in the aircraft. As such, the Braggwavelength associated with each zone differs from the Bragg wavelengthassociated with the other zones. Because interrogator 16 a can identifywhich Bragg wavelength is associated with which zone, interrogator 16 amay determine the distance to each FBG based on the time taken for theBragg wavelength to travel from first end 28 a, to the FBG, and back tofirst end 28 a. The Bragg wavelength is sensitive to both strain andtemperature. Changes in strain and temperature result in a shift in theBragg wavelength, which can be detected by interrogator 16 a and used todetermine the change in strain and/or temperature.

In WDM, interrogator 16 a provides an optical signal to first end 28 aof optical fiber 18 a with optical transmitter 20 a. Optical transmitter20 a can be a tunable, swept-wavelength laser. The wavelength of opticaltransmitter 20 a is swept across a pre-defined range. The wavelength ofthe optical signal being transmitted at any given moment in time isknown. The Bragg wavelengths are received at first end 28 a of opticalfiber 18 a by detector 22 a, and interrogator 16 a correlates or mapschanges in the Bragg wavelengths into intensity as a function of time. Ashift in the Bragg wavelength indicates a change in temperature and/orstrain, and tracking the changes in the Bragg wavelength enablesinterrogator 16 a to determine the temperature at each FBG within eachzone Z₁-Z_(n).

In TDM, optical transmitter 20 a is a broadband laser light source suchthat multiple wavelengths are transmitted through optical fiber 18 a.Each FBG is configured to reflect a particular Bragg wavelength.Interrogator 16 a monitors the time required for the each Braggwavelength to return to first end 28 a. The time required for each Braggwavelength to return to first end 28 a indicates the location of eachFBG in optical fiber 18 a. Having established the location of each FBGin optical fiber 18 a, optical transmitter 20 a provides pulses throughoptical fiber 18 a. The wavelength of each pulse can be determined whenthe reflected pulse arrives at interrogator 16 a. Changes in thewavelength are detected and converted to intensity verses time, therebyallowing interrogator 16 a to determine the temperature at the locationof each FBG in optical fiber 18 a.

In WDM/TDM, interrogator 16 a provides optical signals through opticalfiber 18 a utilizing both a tunable, swept-wavelength laser and abroadband laser light source. Similar to both WDM and TDM, in WDM/TDMthe reflected Bragg wavelengths are monitored for any changes in thewavelengths. The changes in the wavelengths are converted to intensityverses time, thereby allowing interrogator 16 a to determine thetemperature at the location of each FBG. WDM/TDM reduces the loss of anysignal in the FBG and the total wavelength that must be scanned tointerrogate the Bragg wavelength is similarly reduced. Temperaturechanges cause the Bragg wavelength to shift, and the shift in the Braggwavelength is analyzed by interrogator 16 a to determine the temperatureshift, and thereby whether an overheat event has occurred. In addition,the location of the overheat event is detected by interrogator 16 abased on the shift in a particular Bragg wavelength, as the location ofa FBG associated with a Bragg wavelength is known.

In some non-limiting embodiments, interrogator 16 a can analyze theoptical signal using any suitable method, including Optical Time DomainReflectometry (OTDR), COFDR, Brillouin Optical Frequency Domain Analysis(BOFDA), Brillouin Optical Time Domain Analysis (BOTDA), IncoherentOptical Frequency Domain Reflectometry (IOFDR) utilizing a SweptFrequency Methodology, and IOFDR utilizing a Step Frequency Methodology.Examples of such methods can be found in co-pending U.S. patentapplication Ser. No. 15/600,100 filed on May 19, 2017, which is hereinincorporated by reference in its entirety.

Existing overheat detection sensors and systems are based on atechnology using eutectic salts as a temperature switch to indicate whena leak occurs in the system, e.g., a bleed air system. The eutectic saltsensor technology however, is reaching the limitations of its capabilitywith respect to manufacturability, precision of overheat detection,overheat location, and fault location. Additionally, rapid changes havebeen seen in the overheat detection system industry requirements, e.g.,aircraft industry, which, because of the reduced tolerance of compositesto increased ambient temperature, require rapid detection of relativelysmall overheat events. The net result is a need to look for an alternatetechnical solution to address this need.

A candidate for the next generation overheat detection system is basedon the above mentioned distributed temperature sensing using FBGs. A FBGis an optical sensor consisting of periodic index of refraction changeswithin the core of a single-mode optical fiber. The FBG acts as awavelength selective mirror, reflecting only in a narrow wavelengthband, which varies with strain and/or temperature experienced by theoptical fiber. Measurements are then made by determining the amount ofshift of the center wavelength of the reflected signal.

As discussed above, an interrogator connected to the optical fiber withFBGs will use either a scanned wavelength laser or a broadband sourcewith a spectrum analyzer to generate a signal representing a returnedspectrum from the sensing array of FBGs. For a single FBG, the returnspectrum is a narrow Gaussian shaped return, the center wavelength ofwhich is dependent on temperature and strain of the location on opticalfiber where the single FBG is located. A significant advantage of asystem involving FBGs is that there are two options for multiplexinglarge sensor arrays into a single interrogator: wavelength divisionmultiplexing (WDM); and time division multiplexing (TDM).

For a WDM system, the FBGs can be fabricated in well-defined wavelengthzones, where each zone is independent. The return spectrum for a WDMtype system has characteristic Gaussian returns spaced across thespectrum, each return representing a unique FBG. A limit or constraintof such a system is the amount of spectrum that can be interrogated andthe amount of spectral movement expected during the measurement for eachFBG. N some non-limiting embodiments, systems can scan a laser over 40nm with 16 defined zones, each of which can monitor a sensor over a 200°C. temperature range. The relative movement of the wavelength center foran FBG with respect to temperature is typically around 10 pm/° C.

For a TDM system, the signal source is pulsed with very short pulses.The concept is to differentiate unique FBGs in a single optical fiber bythe time it takes the reflected optical signal to return from each FBG.Representative time values are about 1 nanoseconds for 10 centimeters ofoptical fiber length. So, to measure FBG sensors spaced 0.5 meters aparton an optical fiber, the optical signal pulse should not be greater than5 nanoseconds in width. To ensure the reflected optical return signalrepresents only one FBG sensor at a given time, a pulse around half thewidth of 5 nanoseconds would be beneficial, such as for example, arelationship of 0.5 nanoseconds per 10 centimeters of fiber length. Foran overheat application, dual-sided interrogation can be used to monitorup to multiple independent channels, each with a number of zonesseparated in wavelength and including a specific wavelength zone thatwill use TDM to provide quasi-distributed temperature measurements.Representation of this concept is depicted in FIGS. 4A and 4B.

Method to Isolate Individual Channels in a Multi-Channel Fiber OpticEvent Detection System (FIGS. 4A-6)

The next portions of the disclosure refer to and discuss a method toisolate individual channels in a multi-channel fiber optic eventdetection system.

FIG. 4A is a simplified block diagram of first LRU 52 a (linereplaceable unit), second LRU 52 b, and third LRU 52 c and shows firstinterrogator 16 a, second interrogator 16 b, and first, second, andthird LRUs 52 a, 52 b, and 52 c respectively including: optical fibers18 a ₁, 18 a ₂, and 18 a ₃; first connectors 54 a, 54 b, and 54 c;second connectors 56 a, 56 b, and 56 c; overheat FBG sensors 58 a, 58 b,and 58 c; temperature FBG sensors 60 a, 60 b, and 60 c; and breaks 62 a,62 b, and 62 c in optical fibers 18 a, 18 b, and 18 c). First, second,and third LRUs 52 a, 52 b, and 52 c and the components thereof aresubstantially similar, and for purposes of clarity and ease ofdiscussion, first LRU 52 a will be discussed in further detail. In thenon-limiting embodiment shown in FIG. 4A, breaks 62 a, 62 b, and 62 care shown as being present in first LRU 52 a, second LRU 52 b, and thirdLRU 52 c. However, breaks 62 a, 62 b, and 62 c are typically notincluded in first LRU 52 a, second LRU 52 b, and third LRU 52 c, butrather it should be understood that breaks 62 a, 62 b, and 62 crepresent potential physical conditions of first LRU 52 a, second LRU 52b, and third LRU 52 c that can form and/or be present.

First LRU 52 a is a discrete line replaceable unit that is part ofoverheat detection system 10 (shown in FIG. 1). First LRU 52 a includesfirst connector 54 a, second connector 56 a, and optical fiber 18 a ₁.First connector 54 a and second connector 56 a are linking devices.Overheat FBG sensors 58 a are fiber Bragg grating (“FBG”) opticalsensors configured to sense an overheat condition of optical fiber 18 a₁. In this non-limiting embodiment, three overheat FBG sensors 58 a areshown to be positioned between consecutive temperature FBG sensors 60 a.In other embodiments, there can be more or less than three consecutiveoverheat FBG sensors 58 a positioned between consecutive temperature FBGsensors 60 a, such as for example twenty overheat FBG sensors 58 a.

Temperature FBG sensors 60 a are FBG optical sensors configured to sensea temperature of optical fiber 18 a ₁. In other non-limitingembodiments, quantities of overheat FBG sensors 58 a and temperature FBGsensors 60 a included in first LRU 52 a can be more or less than thequantities shown in FIGS. 4A and 4B. In this non-limiting embodiment,approximately uniform distances are shown between adjacent same-typeFBGs along optical fiber 18 a ₁, however, non-uniform distances can alsobe incorporated. Break 62 a is a breakage or damaged portion in opticalfiber 18 a ₁. In this non-limiting embodiment, break 62 a represents apotential physical state of a portion of optical fiber 18 a ₁. Forexample, the typical operating state of optical fiber 18 a ₁ does notinclude break 62 a (and likewise for optical fibers 18 a ₂ and 18 a ₃).

First LRU 52 a is attached and connected to first and secondinterrogators 16 a and 16 b via first and second connectors 54 a and 56a. First connector 54 a is mounted onto an end of optical fiber 18 a ₁and is connected to first interrogator 16 a. Second connector 56 a ismounted onto the opposite end of optical fiber 18 a ₁ from firstconnector 54 a and is connected to second interrogator 16 b. OverheatFBG sensors 58 a and temperature FBG sensors 60 a are disposed in andalong portions of optical fiber 18 a ₁. Break 62 a can be disposed in aportion of optical fiber 18 a ₁.

In this non-limiting embodiment, first interrogator 16 a functions asthe primary, or master, interrogator with second interrogator 16 bfunctioning as the secondary, or slave, interrogator. For example,second interrogator 16 b will typically occupy a ready state, but willnot actively interrogate optical fiber 18 a ₁ unless required to do sofor system testing or in the event one of the FBGs breaks and the entirelength of optical fiber 18 a ₁ can no longer be interrogated from oneend. In a breakage event (e.g., formation of break 62 a), secondinterrogator 16 b is activated to inspect broken optical fiber 18 a ₁from the opposite side of break 62 a as from first interrogator 16 a.

First LRU 52 a provides a replaceable segment of optical fiber to beused in overheat detection system 10. First connector 54 a attaches andconnects optical fiber 18 a ₁ to first interrogator 16 a. Secondconnector 56 a attaches and connects optical fiber 18 a ₁ to secondinterrogator 16 b. Overheat FBG sensors 58 a reflect a specific range ofwavelength of light in order to detect if an overheat condition ispresent at the locations of each of overheat FBG sensors 58 a alongoptical fiber 18 a ₁. Temperature FBG sensors 60 a reflect a specificrange of wavelength of light in order to sense a current temperature ofthe locations of each of overheat FBG sensors 58 a along optical fiber18 a ₁. Break 62 a is the result of, e.g., physical trauma, fatigue, orother damage experienced by optical fiber 18 a ₁ and has the effect ofcorrupting or blocking an optical signal sent through optical fiber 18 a₁.

Incorporating several and separate LRUs into overheat detection system10 enables sensing and detecting throughout various regions of aircraft12. Separating the optical fiber into separate LRUs also enables ease ofreplacement of individual LRUs as compared to the possible need toremove the entirety of an optical fiber in an overheat detection systemthat uses a single optical fiber for all zones of aircraft 12.Additionally, the dual-interrogator configuration depicted in FIG. 4Aenables optical fiber 18 a ₁ to be optically probed from both ends ofoptical fiber 18 a ₁. This capability and functionality is beneficialbecause if optical fiber 18 a ₁ becomes damaged and sustains, e.g.,break 62 a, optical signals can be sent from either side of break 62 a.Accordingly, techniques of this disclosure can enable overheat detectionsystem 10 to gather data from the FBGs located on both sides of break 62a, rather than a single side as in a configuration incorporating only asingle interrogator on one end of the optical fiber.

FIG. 4B is a simplified block diagram of left LRU 52L and right LRU 52Rand shows first interrogator 16 a, second interrogator 16 b, left LRU52L (including optical fiber 18L, first connector 54L, second connector56L, overheat FBG sensors 58L, and temperature FBG sensors 60L), andright LRU 52R (including optical fiber 18R, first connector 54R, secondconnector 56R, overheat FBG sensors 58R, and temperature FBG sensors60R, and break 62 in optical fiber 18R). Left LRU 52L and right LRU 52Rare substantially similar to first LRU 52 a from FIG. 4A. In FIG. 4B,left LRU 52L and right LRU 52R are connected to each other in anend-to-end arrangement. Left second connector 56L of left LRU 52L isconnected to right first connector 54R of right LRU 52R. In thisnon-limiting embodiment, two consecutive LRUs are shown connected inseries. In other non-limiting embodiments, more than two LRUs can beconnected consecutively and serially to form a chain of multiple LRUsthat can extend throughout various or all zones of aircraft 12.

FIG. 5A is a block diagram of interrogator 16 a and shows interrogator16 a (with optical transmitter 20 a, detector 22 a, couplers 26(including first tier coupler 64, second tier couplers 66 a and 66 b,and third tier couplers 68 a, 68 b, and 68 c), detectors 70 a, 70 b, and70 c, and optical switches 72 a, 72 b, and 72 c) and first, second, andthird optical fibers 18 a, 18 b, and 18 c (with respective firstconnectors 54 a, 54 b, and 54 c).

Interrogators 16 a and 16 b (shown in FIGS. 4A and 4B) are substantiallysimilar, and for ease of discussion, interrogator 16 a with opticaltransmitter 20 a, detector 22 a, and computer-readable memory 24 a willbe discussed in further detail with reference to FIG. 5A. First tiercoupler 64, second tier couplers 66 a and 66 b, and third tier couplers68 a, 68 b, and 68 c are optical devices with one or more optical inputsand one or more optical outputs, and which are capable of splitting anoptical signal into multiple channels. Detectors 70 a, 70 b, and 70 care receivers configured to receive an optical signal. Optical switches72 a, 72 b, and 72 c are in-line devices that are configured toselectively block optical signals.

Controller 14 (shown in FIG. 1) is operatively connected to interrogator16 a, such that optical transmitter 22 a and switches 72 a, 72 b, and 72c receive signals from controller 14 and detectors 22 a, 70 a, 70 b, and70 c send signals to controller 14. First tier coupler 64 is disposed infirst interrogator 16 a and is optically connected to opticaltransmitter 20 a, to detector 22 a, and to second tier couplers 66 a and66 b. Second tier coupler 66 a is disposed in first interrogator 16 aand is optically connected to first tier coupler 64 and to third tiercouplers 68 a and 68 b. Second tier coupler 66 b is disposed in firstinterrogator 16 a and is optically connected to first tier coupler 64and to third tier coupler 68 c. Third tier coupler 68 a is disposed infirst interrogator 16 a and is optically connected to second tiercoupler 66 a, to detector 70 a, and to optical switch 72 a. Third tiercoupler 68 b is disposed in first interrogator 16 a and is opticallyconnected to second tier coupler 66 a, to detector 70 b, and to opticalswitch 72 b. Third tier coupler 68 c is disposed in first interrogator16 a and is optically connected to second tier coupler 66 b, to detector70 c, and to optical switch 72 c.

Detector 70 a is disposed in first interrogator 16 a and is opticallyconnected to third tier coupler 68 a. Detector 70 b is disposed in firstinterrogator 16 a and is optically connected to third tier coupler 68 b.Detector 70 c is disposed in first interrogator 16 a and is opticallyconnected to third tier coupler 68 c. Optical switch 72 a is disposed infirst interrogator 16 a and is optically connected to third tier coupler68 a and to first connector 54 a. Optical switch 72 b is disposed infirst interrogator 16 a and is optically connected to third tier coupler68 b and to first connector 54 b. Optical switch 72 c is disposed infirst interrogator 16 a and is optically connected to third tier coupler68 c and to first connector 54 c. In this non-limiting embodiment,optical switches 72 a, 72 b, and 72 c are disposed downstream fromcouplers 26 (with a downstream direction flowing from opticaltransmitter 20 a in a left to right direction as shown in FIG. 5A). Inthis non-limiting embodiment, optical switches 72 a, 72 b, and/or 72 cfor channel isolation are needed for a dual-ended interrogationconfiguration specifically.

In this non-limiting embodiment, detector 22 a is used for a TDM portionof overheat detection system 10. First tier coupler 64, second tiercouplers 66 a and 66 b, and third tier couplers 68 a, 68 b, and 68 csplit optical signals originating from optical transmitter 20 a anddistribute the split optical signals to optical fibers 18 a, 18 b, and18 c. First tier coupler 64, second tier couplers 66 a and 66 b, andthird tier couplers 68 a, 68 b, and 68 c are also configured to receivemultiple return signals from optical fibers 18 a, 18 b, and 18 c andmerge the return signals into a single channel connected to detector 22a.

Detectors 70 a, 70 b, and 70 c detect optical signals received fromindividual optical fibers 18 a, 18 b, and 18 c. In this non-limitingembodiment, detectors 70 a, 70 b, and 70 c are used for a WDM mode foreach of optical fibers 18 a, 18 b, and 18 c. Optical switches 72 a, 72b, and 72 c selectively block optical signals from passing acrossoptical switches 72 a, 72 b, and 72 c. Optical switches 72 a, 72 b, and72 c are controlled to turn off each channel independently at firstinterrogator 16 a (and likewise at second interrogator 16 b with similaror identical components).

In a dual-interrogator configuration (as shown in FIGS. 4A and 4B) withboth interrogators scanning at the same time, simultaneous operation ofboth interrogators can result in difficulty in measuring the reflectedsignals from the FBG chain due to the multiple signals crossing-over oneach respective channel (or optical fiber). If one of optical fibers 18a, 18 b, or 18 c sustains a break, second interrogator 16 b can beactivated due to the break in the optical fiber preventing the opticalsignal from reaching the far end of the optical fiber. However, thechannels, or optical fibers, that are not broken will have the problemof seeing the optical signal from the second interrogator as there isnothing stopping the cross talk from the second optical signal. As such,additional isolation of the channels is preferable to enable dual endedinterrogation for these types of systems.

Optical switches 72 a, 72 b, and 72 c can be controlled to turn off eachchannel (e.g., optical fibers 18 a, 18 b, and 18 c) independently, andat each of first and second interrogators 16 a and 16 b. Such aconfiguration of first interrogator 16 a with optical switches 72 a, 72b, and 72 c allows for the use of a single laser (e.g., opticaltransmitter 20 a) while also providing channel independence between eachof optical fibers 18 a, 18 b, and 18 c. Optical switches 72 a, 72 b, and72 c are independently controlled to allow channel isolation as needed.In one non-limiting embodiment, if first interrogator 16 a detects thatan optical fiber is open (e.g., damaged, or otherwise not transmitting asignal), second interrogator 16 b will awake from a standby mode inresponse to a communication from first interrogator 16 a. Only the openoptical fiber will be interrogated (i.e., illuminated by opticaltransmitter 20 b in second interrogator 16 b), while signals through theremaining optical fibers will be controlled (i.e., blocked) by opticalswitches 72 a, 72 b, and/or 72 c.

FIG. 5B is a block diagram of first interrogator 16 a with opticalswitches 72 a, 72 b, and 72 c positioned upstream of third tier couplers68 a, 68 b, and 68 c. FIG. 5B shows interrogator 16 a (with opticaltransmitter 20 a, detector 22 a, couplers 26 (including first tiercoupler 64, second tier couplers 66 a and 66 b, and third tier couplers68 a, 68 b, and 68 c), detectors 70 a, 70 b, and 70 c, and opticalswitches 72 a, 72 b, and 72 c) and first, second, and third opticalfibers 18 a, 18 b, and 18 c (with respective first connectors 54 a, 54b, and 54 c). In FIG. 5B, optical switches 72 a, 72 b, and 72 c aredisposed between second tier couplers 66 a and 66 b and third tiercouplers 68 a, 68 b, and 68 c. This configuration is different than theconfiguration in FIG. 5A that includes third tier couplers 68 a, 68 b,and 68 c disposed between second tier couplers 66 a and 66 b and opticalswitches 72 a, 72 b, and 72 c.

The alternate configuration shown in FIG. 5B, (i.e., having opticalswitches 72 a, 72 b, and 72 c located upstream of third tier couplers 68a, 68 b, and 68), enables the individual detectors 70 a, 70 b, and 70 cto be used as monitors for the optical signals transmitted by theopposite interrogator, which in this non-limiting embodiment is secondinterrogator (e.g., as shown in FIGS. 4A and 4B).

FIG. 6 is a block diagram of first interrogator with optical switch 72configured as a 1×N optical switch. FIG. 6 shows interrogator 16 a (withoptical transmitter 20 a, detector 22 a, coupler 26, and opticalswitches 72) and first, second, and third optical fibers 18 a, 18 b, and18 c (with respective first connectors 54 a, 54 b, and 54 c). In thisnon-limiting embodiment, optical switch 72 includes a 1×3 opticalswitch. In other non-limiting embodiments, optical switch 72 can includea 1×N optical switch, wherein N can equal more or less than 3 outputchannels. Optical switch 72 as shown in FIG. 6 provides an alternateconfiguration from those shown in FIGS. 5A and 5B that enable only onechannel to transmitted and received at a given time (e.g., one ofoptical fibers 18 a, 18,b or 18 c to receive a signal at a time).

Controller 14 (shown in FIG. 1) is operatively connected to interrogator16 a such that optical transmitter 22 a and switch 72 receive signalsfrom controller 14 and detector 22 a sends signals to controller 14.With communication between first and second interrogators 16 a and 16 b,first and second interrogators 16 a and 16 b (each with respective 1×Noptical switches) can both cycle through the channels without eversimultaneously transmitting on the same channel at the same time,thereby resulting in a slower overall update rate, but requiring lesscomponents and providing a significantly better power efficiency

In one non-limiting embodiment, in order to scan (i.e., reflect lightby) individual temperature FBGs 60 (shown in FIGS. 4A and 4B), pulsedlaser light can be used. The pulse duration is short enough thatdetector 22 a only sees return signal responses from one FBG at a time.Typically, this means that the pulse duration is less than half of thetime required to travel round trip (i.e., from first interrogator 16 a,to a particular FBG, and back to first interrogator 16 a) to the nextFBG in line versus the current sensor in line. In one non-limitingembodiment, a round trip time to a FBG sensor can equate to 1 nanosecondper 10 centimeters of optical fiber length. For example, for aseparation distance of 0.5 meters, the time equates to 5 nanoseconds,indicating that the pulse duration of the optical signal should be halfthat time duration (e.g., 5 nanoseconds) or less. Return responsesignals are more easily identifiable as the response signals drop tozero in between the sensor returns.

If the separation between sensors and the pulse timing is correctlyidentified, a typical approach would be to sample the return signal fromdetector 22 a using an analog-to-digital converter and measure thetiming the sampling rate to match the round trip time between FBGsensors. For example, for a 0.5 meter separation distance equating to a5 nanosecond round trip time, an example sample rate would be 200megahertz. Such a sampling rate would provide one sample value for eachFBG sensor. An important part of the sampling is that the timing is suchthat the center of the sampling matches the time at which the pulse iscentered on the sensor. If the timing is off, the response signal may besampled during the rising edge or falling edge of the pulse, or worseyet, at that time where there is no return between pulses.

For most systems, this is a trivial problem as the timing can be definedby the distance to the start of the first FBG sensor and then repeatwith equidistant sensors. In most cases, the timing for the first FBGsensor can also be defined in a calibration table in the software of aninterrogator. For a non-limiting fiber optic overheat system such asoverheat detection system 10, a first design criterion prevents theupdating of any calibration tables after overheat detection system 10 isinstalled. In this non-limiting embodiment, a second design criterion isthat overheat detection system 10 may require between six and ten LRUsections, each connected serially to the next using connectors (e.g.,first and second connectors 54 and 56). Given that calibration of thetiming between sensors may be prohibited after overheat detection system10 is installed on aircraft 12, some options are available.

Timing Markers for Fiber Sensing Systems (FIGS. 7-8)

The next portions of the disclosure refer to and discuss timing markersfor fiber sensing system.

FIG. 7 is a simplified block diagram of LRU 52 and shows firstinterrogator 16 a, second interrogator 16 b, and first LRU 52 (includingoptical fibers 18 a, first connector 54, second connector 56, overheatFBG sensors 58, temperature FBG sensors 60, and timing FBG sensors 74).LRU 52 shown in FIG. 7 is substantially similar to first LRU 52 a shownin FIG. 4A, and so the discussions of the components of first LRU 52 afrom FIG. 4A also applies to LRU 52 shown in FIG. 7. LRU 52 additionallyincludes timing FBG sensors 74. Timing FBG sensors 74 are fiber Bragggrating optical sensors configured to reflect an optical signal.

Timing FBG sensors 74 are disposed in and along portions of opticalfiber 18 a ₁. In this non-limiting embodiment, a timing FBG sensor 74 isdisposed between first connector 54 a and a temperature FBG sensor 60that is nearest to first connector 54 a. Also in this non-limitingembodiment, another timing FBG sensor 74 is disposed between secondconnector 56 a and a temperature FBG sensor 60 that is nearest to secondconnector 56 a. In other non-limiting embodiments, there can be more orless than two temperature FBG sensors 60 disposed along LRU 52. In thisnon-limiting embodiment, timing FBG sensors 74 are needed formultiplexed, in some cases highly multiplexed, TDM type systems. Inother non-limiting embodiments, timing FBG sensors 74 can be used witheither a single interrogator or a dual interrogator (interrogator onboth ends) type of design.

Timing FBG sensors 74 are disposed in and along portions of opticalfiber 18 a ₁ reference locations of optical fiber 18 a ₁. Duringoperation of overheat detection system 10, optical transmitter 22 a(shown in FIGS. 5A-6) emits a first optical signal into optical fiber 18a via first interrogator 16 a. The first optical signal is reflected byone of timing FBG sensors 74 to create a response signal. The responsesignal is received by detector 22 a in first interrogator 16 a fromoptical fiber 18 a based upon the reflected first optical signal. Theresponse signal is received by detector 22 a after a first amount oftime that defines a first time step and a first rate of the responsesignal. The distance from the first interrogator to the first timingfiber Bragg grating is detected. The response signal is sampled at asampling rate that is greater than the first rate of the responsesignal. Sampling the response signal includes measuring the amount ofthe response signal with detector 22 a to create sample response ratevalues.

The sample response rate values are compared to the response signal toidentify which of the sample response rate values correspond with alocal maximum of the response signal. (See e.g., FIG. 8 and relateddiscussion). The distance from the first interrogator to the firsttiming fiber Bragg grating can be determined from the comparison of thesample response rate values with the detected response signal. Forexample, controller 14 (shown in FIG. 1) is operatively connected tofirst interrogator 16 a and is configured to determine the referencelocations of temperature FBG sensors 60 of optical fiber 18 a. Overheatdetection system 10 with temperature FBG sensors 60 enables first andsecond interrogators 16 a and 16 b to detect distances to specifictiming FBG sensors 74 for each section of optical fiber 18 a and adjustthe sampling timing (or use an oversampling method) to ensure thattiming of the sampling coincides with the centers of the return pulsesfrom timing FBG sensors 74 along optical fiber 18 a. Overheat detectionsystem 10 with temperature FBG sensors 60 adds additional FBG sensors ineach sensing length of optical fiber 18 a that act as timing markers toallow overheat detection system 10 to self-calibrate the timing neededto properly interrogate the sensor chains.

In order to align the sampling of the response signal with the timing ofthe response signal, the return signal is oversampled (sample at ahigher rate) and the samples that line up with the timing of the returnpulses for that section of the return signal are analyzed. FIG. 8 showsa depiction of this option.

FIG. 8 shows graph 76 including a depiction of output signal 78 frominterrogator 16 a and a series of sampling points of a return signal.FIG. 8 shows graph 76, output signal 78, first channel Ch1, secondchannel Ch2, third channel Ch3, first clock cycle 1, second clock cycle2, third clock cycle 3, fourth clock cycle 4, first channel pulses 80,second channel pulses 82, and third channel pulses 84.

Graph 76 is a graphical representation of measures of luminous flux forsignals correlating to output signal 78, first channel Ch1, secondchannel Ch2, and third channel Ch3 relative to first clock cycle 1,second clock cycle 2, third clock cycle 3, and fourth clock cycle 4.Output signal 78 is an optical signal sent from interrogator 16 a (e.g.,emitted by optical transmitter 20 a) and distributed into optical fiber18 a. First channel Ch1, second channel Ch2, and third channel Ch3 arerepresentative of separate optical fibers such as optical fibers 18 a,18 b, and 18 c. First clock cycle 1, second clock cycle 2, third clockcycle 3, and fourth clock cycle 4 are sequential time steps that repeatevery four steps. First channel pulses 80, second channel pulses 82, andthird channel pulses 84 are representative of detected amounts of light(i.e., reflected return signals from optical fibers 18 a, 18 b, and 18c) measured by one of detectors 70 a, 70 b, and 70 c.

Output signal 78 is positioned on a left side of graph 76 to indicatethat the start of output signal coincides with first (e.g., left-most)clock cycle 1. An amplitude, or height, and shape of output signalcorrespond to the amount of light and periodic nature of output signal78 as the output signal is created and distributed into optical fibers18 a, 18 b, and 18 c. First channel Ch1, second channel Ch2, and thirdchannel Ch3 represent reflected response signals from FBG sensorsdisposed on optical fibers 18 a, 18 b, and 18 c. In this non-limitingembodiment, first channel Ch1, second channel Ch2, and third channel Ch3correspond to optical fibers 18 a, 18 b, and 18 c. In other non-limitingembodiments, more or less than three channels can be sensed.

First clock cycle 1, second clock cycle 2, third clock cycle 3, andfourth clock cycle 4 are sequential time periods that are of equalduration. First channel pulses 80, second channel pulses 82, and thirdchannel pulses 84 are shown as being assigned to their respectivechannels (e.g., Ch1, Ch2, and Ch3). In relation to overheat detectionsystem 10, first channel pulses 80, second channel pulses 82, and thirdchannel pulses 84 correspond to detected return signals from each offiber optics 18 a, 18 b, and 18 c. The size, shape, and spacing of firstchannel pulses 80, second channel pulses 82, and third channel pulses 84are analyzed to determine sample response rate values. As shown in FIG.8, first channel pulses 80, second channel pulses 82, and third channelpulses 84 are shown as being shifted 90 a multiple of discrete clockcycles (i.e., representing multiples of a 90° or a π/2 phase shift).

A method of spatially synchronizing a series of timing FBG sensors 74disposed on optical fibers 18 a, 18 b, and 18 c includes emitting, byoptical transmitter 20 a, a first optical signal (e.g., output signal78) into optical fibers 18 a, 18 b, and 18 c. The first optical signalis reflected by timing FBG sensors 74 to create response signals. Theresponse signals are received by detector 22 a from optical fibers 18 a,18 b, and 18 c based upon the reflected first optical signal. Theresponse signals are received by detector 22 a after a first amount oftime that defines a first time step and a first rate of the responsesignals. The response signal is sampled at a sampling rate that isgreater than the first rate of the response signal. Sampling theresponse signal comprises measuring the amount of the response signalwith detector 22 a (or by detectors 70 a, 70 b, or 70 c) in firstinterrogator 16 a to create sample response rate values (i.e., measuredfrom first channel pulses 80, second channel pulses 82, and thirdchannel pulses 84). The sample response rate values are compared to theresponse signals to identify which of the sample response rate valuescorrespond with local maximums of the response signals. From thiscomparison, the distance from first interrogator 16 a to timing FBGsensors 74 can be detected, calculated, or determined.

For example, a sampling rate can include a rate larger than the rate ofthe response signal by a factor of four, so for a non-limitingembodiment with a response signal rate at 200 megahertz, a sampling of800 megahertz could be used. Such a sampling rate would provide foursamples for each required time step. In FIG. 8, the timing of when thefour samples are measured/detected is represented by first clock cycle1, second clock cycle 2, third clock cycle 3, and fourth clock cycle 4.Depending on where the pulse fell within the timing windows, thosesamples could see no light, see light from the rising or falling edge ofthe pulse, or see light from the peak of the pulse. If the pulse isroughly half the width of the timing step, at least two of the sampleswould fall in the ‘peak’ zone of the pulse. The timing marker wouldindicate exactly which of the samples lined up for that given sensingsection. Each section would have its own ‘calibration’ coefficient thatsimply represents which of the samples (1 through 4) is used for thatsection of optical fiber 18 a.

Timing FBG sensors 74 (e.g., as timing markers) allow for somerelaxation of the manufacturing requirements for the sensing lengths,and especially the length between first connector 54 a and the firsttemperature FBG sensor 60. Timing FBG sensors 74 effectively communicateto overheat detection system 10 where the start and finish for each LRUare in time, and such that overheat detection system 10 could ignore thespace in between. Using timing FBG sensors 74 in this manner alsoenables the LRUs mostly immune to which end is considered front andwhich is back. Overheat detection system 10 is able to locate each oftiming FBG sensors 74 and adjust for either installation direction. Forthe dual-interrogator configuration (e.g., overheat detection system 10including first and second interrogators 16 a and 16 b), each of firstand second interrogators 16 a and 16 b can conduct its own calibrationmeasurement of optical fiber 18 a and timing FBG sensors 74 would beseen in the opposite order, and first and second interrogators 16 a and16 b can develop their own unique calibration numbers.

In one non-limiting embodiment, first and second interrogators 16 a and16 b can be placed in their own respective wavelength channel in a WDMscheme. To ease the calibration, a broad spectral return FBG sensorcould be incorporated into LRU 52 (or any of LRUs 52 a, 52 b, or 52 c)so that a single wavelength could locate each timing FBG sensors 74 intime regardless of the temperature of those timing FBG sensors 74 (i.e.,a center wavelength of an FBG shifts with temperature).

Timing FBG sensors 74 can also act as a type of bit to ensure that thevarious LRUs are installed in the correct locations (i.e., mistakeproofing). Since the lengths of the LRUs are pre-defined, if the overeatdetection system 10 were to find the separation between two timing FBGsensors 74 to not match the expected distance, an indication could besent that the wrong LRU was installed at a specific location.

Device and Method of Calibrating Fiber Bragg Grating Based Fiber OpticOverheat Systems (FIGS. 9A-9B)

The next portions of the disclosure refer to and discuss aself-calibration method and device for fiber Bragg grating based fiberoptic overheat systems.

In one non-limiting embodiment, a design criterion for overheatdetection system 10 includes the ability to detect an overheat eventwithin 5° Celsius of a threshold defined for each of zones Za-Zj ofaircraft 12. The temperature sensing functionality of overheat detectionsystem 10 also includes a 5° Celsius requirement for accuracy. A typicalFBG sensor has a nominal relationship of 10 picometers of wavelengthshift per degree Celsius. A 5° Celsius accuracy thus requires theability to stay within a 50 picometer window in wavelength to maintainthe 5° Celsius. Existing manufacturing capabilities of FBG sensors areable to write gratings with a center wavelength accuracy of 0.1nanometers or 100 picometers. In other existing techniques, theaccuracies of the center wavelength can be better than 0.1 nanometer, insome instances as low as 0.01 nanometers or 10 picometers. However, inthis non-limiting embodiment, neither of these accuracy values willallow overheat detection system 10 to achieve the requisite temperatureaccuracies without calibrating the sensors in some manner.

In this non-limiting embodiment, a method to auto-calibrate the sensingFBG sensing system is provided that meets a design criterion requiresavoidance of the use of calibration tables each time a FBG sensor LRU isinstalled or replaced.

In this non-limiting embodiment, with the criterion for the sensoraccuracy (e.g., +/−5° Celsius accuracy (i.e., 50 picometer)requirement), existing scales of the manufacturing capability of FBGsensors (e.g., +/−100 picometer center wavelength capability) are notthat far off of the accuracy requirements. Depending on the statisticsof accuracy and manufacturing variation, existing capabilities differ bya factor of 2 to a factor of 8 from the required capabilities. Thisfactor of 2 to 8 eases the calibration requirements, with the factor of8 providing a worst case scenario. If the FBG sensors could be testedafter the FBG sensors are manufactured and annealed to their finalstarting wavelength using a fixed, known temperature bath, a nominalcalibration value for those FBG sensors could be obtained. Using such avalue, there would only be a need to place each FBG sensor in one ofeight buckets (i.e., identification or classification regions) todescribe the starting center wavelength for a FBG sensor. If the nominalcalibration value was conveyed to the interrogator, the interrogatorcould use the nominal calibration value to improve overall accuracy tothe level needed by a specific embodiment.

Overheat detection system 10 with calibration FBG sensors 86 enables amethod to have each of calibration FBG sensors 86 tell first or secondinterrogators 16 a or 16 b what its individual calibration values are sothat overheat detection system 10 can meet the accuracy requirements. Inone non-limiting embodiment, there is an underlying assumption that eachof optical fibers 18 a, 18 b, and 18 c contain FBGs with an overallcenter wavelength variation closer to (i.e., less than) the 10 picometervalue that is provided by manufacturers as a possible variance for asingle optical fiber with a plurality of FBG sensors. The methodincludes conveying a value from 1-8 that represents which bin thestarting wavelength resides in for a particular FBG chain (i.e. aparticular one of optical fibers 18 a, 18 b, or 18 c). These values canbe represented in a 3 bit binary sequence. First interrogator 16 adetects and/or senses that 3 bit sequence from detector 22 a, overheatdetection system 10 can calibrate itself based upon the 3 bit sequence.

FIG. 9A is a simplified block diagram of LRU 52 and shows firstinterrogator 16 a, second interrogator 16 b, and first LRU 52 (includingoptical fibers 18 a, first connector 54, second connector 56, overheatFBG sensors 58, temperature FBG sensors 60, timing FBG sensors 74, andcalibration FBG sensors 86 disposed in a first pattern). LRU 52 shown inFIG. 9A is substantially similar to LRU 52 shown in FIG. 7, and so thediscussions of the components of LRU 52 from FIG. 7 also applies to LRU52 here shown in FIG. 9A. LRU 52 additionally includes calibration FBGsensors 86. FIG. 9B is a simplified block diagram of LRU 52 and showsfirst interrogator 16 a, second interrogator 16 b, and first LRU 52(including optical fibers 18 a, first connector 54, second connector 56,overheat FBG sensors 58, temperature FBG sensors 60, timing FBG sensors74, and calibration FBG sensors 86 disposed in a first pattern). FIGS.9A and 9B are substantially similar, and for ease of discussion will bediscussed mostly in unison (with a portion of the discussion identifyingthe differences between the two).

Calibration FBG sensors 86 are fiber Bragg grating optical sensorsconfigured to reflect an optical signal. Calibration FBG sensors 86 aredisposed in and along portions of optical fiber 18 a ₁. In thenon-limiting embodiment shown in FIG. 9A, calibration FBG sensors 86 arelocated on the ends of optical fiber 18 a and in a position relative tothe other FBG sensors on optical fiber 18A that is closest to firstconnector 54 a and second connector 56 a. Calibration FBG sensors 86 areshown as being disposed adjacent to timing FBG sensors 74. In thenon-limiting embodiment shown in FIG. 9B, calibration FBG sensors 86 arelocated in multiple positions of optical fiber 18 a between overheat FBGsensors 58 and temperature FBG sensors 60. In both of these non-limitingembodiments, there are multiple calibration FBG sensors 86 disposed onoptical fiber 18 a. In other non-limiting embodiments, there can be twoor more calibration FBG sensors 86 disposed on optical fibers 18 a, 18b, and or 18 c.

As shown in FIGS. 9A and 9B, overheat detection system 10 withcalibration FBG sensors 86 uses additional FBGs (i.e., calibration FBGsensors 86) as calibration markers in optical fiber 18 a at setdistances or in set wavelength locations along optical fiber 18 a to actas bits in a 3 bit word (e.g., calibration constant) that first andsecond interrogators 16 a and 16 b interrogator can read to get thecalibration constant. In one non-limiting embodiment, two calibrationconstants are utilized, one calibration constant for overheat FBGsensors 58 and one calibration constant for temperature FBG sensors 60.Alternatively or additionally, if there is room or space in thewavelength or spatial regime to write more bits, the calibrationconstant could consist of 4 or more bits. It is also possible that 2 oreven 1 bit calibration constants will suffice. FIGS. 9A and 9B show howthese concepts can be applied for a system using wavelength bins (oneper bit) or a spatial arrangement in a single wavelength bin where thelocation represents each bit. In the system using spatial arrangement,the spatial location could be referenced to timing FBG sensors 74 thatare used to synchronize the pulse timing for the distributed sensingsystem. In this non-limiting embodiment, overheat detection system 10with calibration FBG sensors 86 can be useful for any FBG system (WDM,TDM, etc.) where there is an opportunity to write additional gratingsinto the sensing fiber that can be used for calibration. In othernon-limiting embodiments, calibration FBG sensors 86 can be used witheither a single interrogator or a dual interrogator (interrogator onboth ends) type of design.

In either of the approaches depicted in the configurations shown inFIGS. 9A and 9B, a binary ‘1’ would indicate when a calibration FBGsensor 86 is present at a reference location and a ‘0’ would indicatewhen there is not a calibration FBG sensor 86 in the reference location.In one non-limiting embodiment, calibration FBG sensors 86 can be usedas part of a WDM configuration. An advantage of a wavelength basedapproach (i.e., WDM) is that WDM enables bi-directional sensing of theoptical fiber. For example, it wouldn't matter from which side opticalfiber 18 a is interrogated, there would be no ambiguity in the values.Since most interrogators have a limit on the wavelengths theyinterrogate across, use of a WDM process could limit the number of zonesthat can be used in overheat detection system 10. In anothernon-limiting embodiment, a spatial process approach only requires onewavelength bin, thus easing the requirement of needing a requisiteamount of available wavelength ranges. However, the direction opticalfiber 18 a is interrogated becomes important. If fiber optic 18 a isinterrogated from different directions, the binary word would appear‘backwards’. To overcome this, an option is to write two timing FBGsensors 74 on one end of optical fiber 18 a and only one timing FBGsensors 74 on the other end of optical fiber 18 a. This use andorientation of timing FBG sensors 74 would define what the forward andbackward directions are.

In one non-limiting embodiment, portions of overheat detection system 10include optical fiber sensing segments (e.g., series of consecutiveLRUs) of approximately 5 meters. In a system that can monitor FBGsensors every 0.5 meters (as may desired in the temperature sensingportion of the system), this allows for as many as eleven calibrationbits in that section. If overheat detection system 10 should requiresignificantly shorter sensing lengths, that requirement could impact theability to create enough bits in that LRU. It is likely that such aninstance would need to be handled with WDM approach. Some combination ofwavelength and spatial distribution is also possible (e.g., WDM, TDM,and/or a combination of WDM and TDM).

In another non-limiting embodiment, a second optical transmitter (e.g.,laser) can be added at a different set of wavelengths to overheatdetection system 10 (e.g., such as adding an L-band laser to a C-bandsystem, etc.). Calibration FBG sensors 86 could be written into opticalfiber 18 a for those new wavelengths from the second opticaltransmitter, thus eliminating a concern of using sensing wavelengths forcalibration. This could add some WDM elements and a second high speeddetector to overheat detection system 10 as well. In anothernon-limiting embodiment, Calibration FBG sensors 86 would be writteninto optical fiber 18 a after any sensing FBGs (e.g., thermal,temperature, and/or timing FBG sensors) are written into optical fiber18 a and annealed (so fixed in wavelength). As such, a manufacturingprocess can include a two-step process with both of those two stepscompleted before any cabling is applied to optic cable 18 a.

In one non-limiting embodiment, calibration information determined byoptical fiber 16 a refers to center wavelengths of each of any of theFBGs (overheat or temperature) in optical fiber 16 a. FBGs (e.g.,overheat FBG sensors 58 a and temperature FBG sensors 60 a) are writteninto optical fiber 16 a with an expected center wavelength designed atsome starting temperature (e.g., 25° Celsius). During operation ofoverheat detection system 10, the center wavelength(s) of the FBGscorresponding to that starting temperature can move around by 0.1 to 0.2nanometers. Since 1° Celsius can cause about 10 picometers of wavelengthshift, that 0.1 to 0.2 nanometer variation can result in errors of 10°to 20° Celsius. As such, calibration FBG sensors 86 can be used tocalibrate overheat detection system 10 by telling overheat detectionsystem 10 something about the starting center wavelength(s) for opticalfiber 16 a. The starting temperature variations are divided into smallerbuckets (e.g., eight buckets) so that the error goes from 10° to 20°Celsius down to 10° to 2.5° Celsius by identifying which of the eightbuckets the center wavelength(s) fell into. In this non-limitingexample, the eight buckets can be described by a three bit word. So, wewould write three calibration FBG sensors 86 into optical fiber 16 athat represent the bits in that word. In another non-limitingembodiment, in order to provide calibration for both overheat andtemperature FBG sensors 58 and 60, there could be a total of threecalibration FBG sensors 86 for each type of overheat and temperature FBGsensors 58 and 60 or six total calibration FBG sensors 86 representingthe calibration bits.

Discussion of Possible Embodiments

The following are non-exclusive descriptions of possible embodiments ofthe present invention.

A system configured to monitor temperature in a plurality of zones of anaircraft includes an optical fiber with first and second ends, first andsecond connectors, and a first interrogator. The optical fiber includesa plurality of fiber Bragg gratings disposed in the optical fiber. Thefirst connector is disposed on the first end of the optical fiber andthe second connector is disposed on the second end of the optical fiber.The first interrogator is connected to the first connector and includesan optical switch. The optical switch is in optical communication withthe first connector of the optical fiber and is configured toselectively block transmission of the optical signal to the opticalfiber to prevent the optical fiber from receiving the optical signalfrom the interrogator.

The system of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components.

A second interrogator can be connected to the second connector of theoptical fiber, wherein the system can be configured to allow monitoringof temperature in the plurality of zones from either of the first orsecond interrogators.

An optical transmitter can be configured to provide an optical signal tothe optical fiber, a first detector can be configured to receive anoptical response from the optical fiber, and/or a coupler can beconnected to the optical transmitter and/or to the detector, wherein thecoupler can be in optical communication with the optical switch.

A controller can be operatively connected to the detector and/or beconfigured to determine at least one temperature for each of theplurality of zones based on the optical response and/or output anindication for detected zones of the plurality of zones in which the atleast one temperature can be greater than a threshold value.

The controller can be configured to control the optical transmitterand/or determine the at least one temperature for each of the pluralityof zones using at least one of time division multiplexing (TDM) andwavelength division multiplexing (WDM).

The aircraft system can be a bleed air system, and wherein the pluralityof zones can comprise bleed air ducts.

The optical transmitter can be configured to provide the optical signalas at least one of a tunable swept-wavelength laser and a broadbandlaser.

A plurality of optical fibers, wherein the first interrogator caninclude a plurality of optical switches, wherein each optical switch cancorresponds to one of each of the optical fibers, wherein the opticalswitches can be configured to control blockage of the optical signalfrom the optical transmitter to the plurality of optical fibers.

The optical fiber can comprise a plurality of line replaceable unitseach including an optical fiber portion, a pair of connectors, and/or aplurality of fiber Bragg gratings that can be disposed on the fiberoptic portion.

A method of detecting thermal conditions for a plurality of zones of anaircraft system includes emitting, by a first optical transmitterdisposed in a first interrogator, a first optical signal. The firstoptical signal is distributed into an optical fiber by a first coupler.The first optical signal is selectively blocked by an optical switch inthe first interrogator from being transmitted into the optical fiber. Asecond optical signal is emitted by a second optical transmitterdisposed in a second interrogator into the optical fiber. A responsesignal based upon the second optical signal is received from the opticalfiber by a second optical receiver in the second interrogator. At leastone temperature, based upon the response signal, for a portion of theplurality of zones is determined using at least one of the first andsecond interrogators.

The method of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingsteps, features, configurations and/or additional components.

The first optical signal can be distributed by a first coupler into aplurality of optical fibers; an optical switch in the first interrogatorcan selectively block the first optical signal from being transmittedinto at least one of the plurality of optical fibers; a second opticaltransmitter disposed in a second interrogator can emit a second opticalsignal into the plurality of optical fibers; a second optical receiverin the second interrogator can receive a response signal from theoptical fibers based upon the second optical signal; and/or a controllercan determine at least one temperature for a portion of the plurality ofzones based upon the response signal.

The optical fiber can include fiber Bragg gratings, and/or whereinemitting, by either the first or second optical transmitters, the firstand second optical signals can comprise emitting the optical signalusing at least one of a tunable, swept-wavelength laser and a broadbandlaser; and/or wherein determining, using the controller, the at leastone temperature for each of the plurality of zones can comprisedetermining the at least one temperature based on at least one of timedivision multiplexing (TDM) and wavelength division multiplexing (WDM).

A first portion of the optical fiber can be monitored with the firstoptical signal up to a break in the optical fiber, wherein the firstportion of the optical fiber can extend from the first interrogator tothe break in the optical fiber; and/or a second portion of the opticalfiber can be monitored with the second optical signal up to the break inthe optical fiber, wherein the second portion of the optical fiber canextend from the second interrogator to the break in the optical fiber.

The first optical switch of the first interrogator and/or a secondoptical switch of the second interrogator can be opened in response to abreak in a portion of the optical fiber, wherein the second opticalswitch can be in optical communication with the optical fiber on an endof the optical fiber opposite from the first interrogator.

A detection system includes an optical fiber, a first connector, asecond connector, a first interrogator, a second interrogator, and acontroller. The optical fiber includes a first end, a second end, and aplurality of fiber Bragg gratings disposed in the optical fiber. Thefirst connector is disposed on the first end of the optical fiber andthe second connector is disposed on the second end of the optical fiber.Each of the first and second interrogators include an opticaltransmitter, a detector, and an optical switch. The optical transmitteris configured to emit an optical signal. The first detector isconfigured to receive an optical response from the optical fiber. Theoptical switch is in optical communication with the optical fiber and isconfigured to selectively block transmission between the optical fiberand both the optical transmitter and the detector to prevent thedetector of one of the first interrogator and the second interrogatorfrom receiving a signal from the optical transmitter of the other of thefirst interrogator and the second interrogator.

The system of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components.

The detection system can be configured to allow the optical switches ofboth the first and second interrogators to allow transmission of anoptical signal when a break in the optical fiber is detected.

The detection system can be configured to be used in an aircraft,wherein the plurality of zones of the optical fiber can relate to aplurality of zones in the aircraft.

The optical fiber, the first connector, and the second connector canmake up a line replaceable unit, wherein the system can comprise aplurality of line replaceable units that can be configured to bedisposed throughout a plurality of zones of the aircraft.

The controller can be configured to control the optical transmitterand/or determine the at least one temperature for each of the pluralityof zones using at least one of time division multiplexing (TDM) andwavelength division multiplexing (WDM).

A plurality of overheat fiber Bragg gratings can be disposed in theoptical fiber; a plurality of temperature fiber Bragg gratings can bedisposed in the optical fiber, wherein the plurality of temperaturefiber Bragg gratings can be interspersed between the plurality ofoverheat fiber Bragg gratings; and/or a first timing fiber Bragg gratingcan be disposed in the optical fiber at a reference location of theoptical fiber.

A system configured to monitor a plurality of zones of an aircraftincludes a first connector, a second connector, an optical fiber, afirst interrogator, and a controller. The first and second connectorsare in optical communication. The optical fiber can extend between thefirst and second connectors, the optical fiber with first and secondends, wherein the first end of the optical fiber is connected to thefirst connector, wherein the optical fiber comprises: a first timingfiber Bragg grating disposed in the optical fiber at a referencelocation of the optical fiber. The first interrogator is connected tothe first end of the optical fiber and is configured to provide a firstoptical signal to the optical fiber and to receive a first timing signalfrom the optical fiber. The first timing fiber Bragg grating isconfigured to provide the first timing signal with information relatedto the first timing fiber Bragg grating. The controller is operativelyconnected to the first interrogator and configured to determine thereference location of the optical fiber based on the first timing signalreceived by first interrogator.

The system of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components.

A plurality of temperature fiber Bragg gratings can be disposed in theoptical fiber.

A second interrogator can be connected to the second end of the opticalfiber, wherein the second interrogator can be configured to provide asecond optical signal to the optical fiber and to receive a secondtiming signal from the optical fiber.

A second timing fiber Bragg grating can be disposed in the opticalfiber, wherein the second timing fiber Bragg grating can be configuredto indicate a second reference location of the optical fiber.

The optical fiber, the first connector, and the second connector canmake up a line replaceable unit, wherein the system can comprise aplurality of line replaceable units disposed throughout the plurality ofzones of the aircraft.

The first timing fiber Bragg grating can be configured to indicate astart point of a line replaceable unit, and wherein the second timingfiber Bragg grating can be configured to indicate a finish point of theline replaceable unit.

A method of spatially synchronizing a series of sensors disposed on anoptical fiber in a system includes emitting, by a first opticaltransmitter disposed in a first interrogator connected to the opticalfiber, a first optical signal into the optical fiber. The optical fiberincludes a plurality of fiber Bragg gratings disposed in the opticalfiber and a first timing fiber Bragg grating disposed in the opticalfiber at a distance from the first interrogator. The first opticalsignal is reflected with the first timing fiber Bragg grating to createa response signal. The response signal is received by a first opticalreceiver in the first interrogator from the optical fiber based upon thereflected first optical signal, wherein the response signal is receivedby the first optical receiver after a first amount of time defining afirst time step and a first rate of the response signal. The responsesignal is sampled at a sampling rate that is greater than the first rateof the response signal. Sampling the response signal includes measuringthe amount of the response signal with a detector in the firstinterrogator to create sample response rate values. The sample responserate values are compared to the response signal to identify which of thesample response rate values correspond with a local maximum of theresponse signal.

The method of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingsteps, features, configurations and/or additional components.

The first optical signal can comprise pulsed laser light.

The distance from the first interrogator to the first timing fiber Bragggrating can be determined.

The plurality of fiber Bragg gratings can comprise a plurality oftemperature fiber Bragg gratings disposed in the optical fiber.

The sampling rate that can be greater than the first rate of theresponse signal by a factor of two or more.

The optical fiber, the first connector, and/or the second connector canmake up a line replaceable unit, wherein the system can comprise aplurality of line replaceable units disposed throughout a plurality ofzones of an aircraft.

A start point of a line replaceable unit can be located based on thesample response rate values, wherein the line replaceable unit cancomprise: a portion of the optical fiber; a first connector can beconnected to a first end of the portion of the optical fiber; a secondconnector can be connected to a second end of the portion of the opticalfiber; a second timing fiber Bragg grating can be configured to indicatea finish point of the line replaceable unit based on the sample responserate values; and/or the finish point of the line replaceable unit can belocated.

An overheat detection system includes first and second connectors inoptical communication, an optical fiber, first and second interrogators,and a controller. The optical fiber extends between the first and secondconnectors and includes first and second ends, with the first end of theoptical fiber is connected to the first connector. The optical fiberincludes a plurality of temperature fiber Bragg gratings, a first timingfiber Bragg grating, and a second timing fiber Bragg grating. The firsttiming fiber Bragg grating is disposed in the optical fiber at areference location of the optical fiber. The second timing fiber Bragggrating is disposed in the optical fiber and is configured to indicate asecond reference location of the optical fiber. The first interrogatoris connected to the first end of the optical fiber and is configured toprovide a first optical signal to the optical fiber and to receive afirst timing signal from the optical fiber. The first timing fiber Bragggrating is configured to provide the first timing signal that includesinformation related to the first timing fiber Bragg grating from thefirst interrogator. The second interrogator is connected to the secondend of the optical fiber and is configured to provide a second opticalsignal to the optical fiber and to receive a second timing signal fromthe optical fiber. The controller is operatively connected to the firstinterrogator and is configured to determine the reference location ofthe optical fiber based on the first timing signal received by the firstinterrogator.

The system of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components.

The detection system can be configured to be installed in an aircraft.

The optical fiber, the first connector, and/or the second connector canmake up a line replaceable unit, wherein the system can comprise aplurality of line replaceable units configured to be disposed throughouta plurality of zones of the aircraft.

The first interrogator can further include: an optical transmitterconfigured to provide the optical signal to the optical fiber; a firstdetector configured to receive a response signal from the optical fiber;and/or a coupler connected to the optical transmitter and/or to thedetector.

An optical switch can be in optical communication with the firstconnector of the optical fiber, wherein the optical switch can beconfigured to selectively block transmission of the optical signal tothe optical fiber.

The controller can be configured to control the optical transmitter anddetermine the at least one temperature for each of the plurality ofzones using at least one of time division multiplexing (TDM) andwavelength division multiplexing (WDM).

A system configured to monitor a plurality of zones of an aircraftincludes a line replaceable unit, a first interrogator, and acontroller. The line replaceable unit includes first and secondconnectors in optical communication and an optical fiber extendingbetween the first and second connectors. The first end of the opticalfiber is connected to the first connector. The optical fibers includes afirst plurality of fiber Bragg gratings disposed in the optical fiberand a plurality of calibration fiber Bragg gratings located in a patternthat provides information related to a calibration value of the linereplaceable unit based upon a center wavelength of each of the firstplurality of fiber Bragg gratings. The first interrogator is connectedto the line replaceable unit at the first end of the optical fiber andis configured to provide a first optical signal to the optical fiber andto receive a first optical response signal from the optical fiber. Thecontroller is operatively connected to the first interrogator and isconfigured to determine the calibration value of the line replaceableunit.

The system of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components.

A plurality of overheat fiber Bragg gratings can be disposed in theoptical fiber.

The plurality of calibration fiber Bragg gratings can be furtherconfigured to indicate a first calibration value, wherein the firstcalibration value can be based upon center wavelengths of the pluralityof overheat fiber Bragg gratings.

A second optical transmitter can be optically connected to the opticalfiber, wherein the second optical transmitter can be configured toprovide a second optical signal to the optical fiber.

The second optical transmitter can be disposed in a second interrogatorconnected to the second end of the optical fiber, wherein the secondinterrogator can be configured to provide the second optical signal tothe optical fiber and to receive a second optical response from theoptical fiber.

The first interrogator can comprise: an optical transmitter configuredto provide an optical signal to the optical fiber; and/or a firstdetector configured to receive an optical response from the opticalfiber, wherein the first detector can be operatively connected to thecontroller.

The system can comprise a plurality of line replaceable units disposedthroughout the plurality of zones of the aircraft.

A plurality of temperature fiber Bragg gratings can be disposed in theoptical fiber, wherein the plurality of temperature fiber Bragg gratingscan be interspersed between the plurality of overheat fiber Bragggratings.

The plurality of temperature fiber Bragg gratings can be furtherconfigured to indicate a second calibration value, wherein the secondcalibration value can be based upon center wavelengths of the pluralityof temperature fiber Bragg gratings.

A method of calibrating a fiber optic overheat system includes emittinga first optical signal into the optical fiber with a first opticaltransmitter disposed in a first interrogator connected to an opticalfiber. The optical fiber includes a plurality of overheat fiber Bragggratings disposed in the optical fiber, and a plurality of calibrationfiber Bragg gratings disposed in the optical fiber. The first opticalsignal is reflected with at least one of the plurality of calibrationfiber Bragg gratings to create a response signal. The response signalfrom the optical fiber based upon the reflected first optical signal isreceived by a first optical receiver in the first interrogator. Thereceived response signal is detected to identify presences of each ofthe plurality of calibration fiber Bragg gratings. A calibration valueis determined based upon the identified presences of the plurality ofcalibration fiber Bragg gratings.

The method of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingsteps, features, configurations and/or additional components.

A plurality of temperature fiber Bragg gratings can be disposed in theoptical fiber, wherein the plurality of temperature fiber Bragg gratingscan be interspersed between the plurality of overheat fiber Bragggratings.

A center wavelength of at least one of the plurality of overheat fiberBragg gratings and the plurality of temperature fiber Bragg gratings canbe identified based upon the detected presences of the plurality ofcalibration fiber Bragg gratings.

A calibration value can be assigned to the line replaceable unit basedupon the detected presences of the plurality of calibration fiber Bragggratings; and/or the calibration value of the line replaceable unit canbe communicated to a controller operatively connected to the opticalreceiver of the first interrogator.

Calibration values for all of the fiber Bragg gratings of the linereplaceable unit can be identified.

A distance from the first interrogator to at least one of the pluralityof calibration fiber Bragg gratings can be determined based upon thecalibration value.

A center wavelength for each of the fiber Bragg gratings can beidentified based upon the calibration value.

A detection system includes a line replaceable unit, a firstinterrogator, a second interrogator, and a controller. The linereplaceable unit includes first and second connectors in opticalcommunication, and an optical fiber extending between the first andsecond connectors. A first end of the optical fiber is connected to thefirst connector. The optical fiber includes a plurality of overheatfiber Bragg gratings, a first timing fiber Bragg grating, and aplurality of calibration fiber Bragg gratings. The first timing fiberBragg grating is configured to indicate at least one of a start pointand end point of the line replaceable unit. The plurality of calibrationfiber Bragg gratings are located in a pattern that provides informationrelated to a calibration value of the line replaceable unit based upon acenter wavelength of each of the first plurality of overheat fiber Bragggratings. The first interrogator is connected to the line replaceableunit at the first end of the optical fiber and is configured to providea first optical signal to the optical fiber and to receive a firstoptical response signal from the optical fiber. The second interrogatoris connected to the second end of the optical fiber and is configured toprovide a second optical signal to the optical fiber and to receive asecond optical response signal from the optical fiber. The controller isoperatively connected to the first interrogator and is configured todetermine the calibration value of the line replaceable unit.

The system of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or additional components.

The detection system can be configured to be installed in an aircraft.

The controller can be configured to control the optical transmitterand/or to determine the at least one temperature for each of theplurality of zones using at least one of time division multiplexing(TDM) and wavelength division multiplexing (WDM).

An optical transmitter can be configured to provide the optical signalto the optical fiber; a first detector can be configured to receive aresponse signal from the optical fiber; and/or a coupler can beconnected to the optical transmitter and to the detector.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

The invention claimed is:
 1. A system configured to monitor a pluralityof zones of an aircraft, the system comprising: a line replaceable unitcomprising: first and second connectors in optical communication; and anoptical fiber extending between the first and second connectors, theoptical fiber with first and second ends, wherein the first end of theoptical fiber is connected to the first connector, wherein the opticalfiber comprises: a first plurality of fiber Bragg gratings disposed inthe optical fiber; and a plurality of calibration fiber Bragg gratingslocated in a pattern that provides information related to a calibrationvalue of the line replaceable unit based upon a center wavelength ofeach of the first plurality of fiber Bragg gratings; a firstinterrogator connected to the line replaceable unit at the first end ofthe optical fiber, wherein the first interrogator is configured toprovide a first optical signal to the optical fiber and to receive afirst optical response signal from the optical fiber; a controlleroperatively connected to the first interrogator and configured todetermine the calibration value of the line replaceable unit based onthe first optical response signal; a second optical transmitterdifferent from the first optical transmitter, wherein the second opticaltransmitter is optically connected to the same optical fiber that thefirst optical transmitter is connected to, wherein the second opticaltransmitter is configured to provide a second optical signal to theoptical fiber, wherein the second optical transmitter is disposed in asecond interrogator different from the first interrogator, wherein thesecond interrogator is connected to the second end of the same opticalfiber that the first interrogator is connected to, wherein the secondinterrogator is configured to provide the second optical signal to theoptical fiber and to receive a second optical response signal from theoptical fiber.
 2. The system of claim 1, wherein the first plurality offiber Bragg gratings comprises: a plurality of overheat fiber Bragggratings disposed in the optical fiber.
 3. The system of claim 2,wherein the plurality of calibration fiber Bragg gratings are furtherconfigured to indicate a first calibration value, wherein the firstcalibration value is based upon center wavelengths of the plurality ofoverheat fiber Bragg gratings.
 4. The system of claim 1, wherein thefirst interrogator further comprises: an optical transmitter configuredto provide an optical signal to the optical fiber; and a first detectorconfigured to receive an optical response signal from the optical fiber,wherein the first detector is operatively connected to the controller.5. The system of claim 1, wherein the system comprises a plurality ofline replaceable units disposed throughout the plurality of zones of theaircraft.
 6. The system of claim 1, wherein the first plurality of fiberBragg gratings further comprises: a plurality of temperature fiber Bragggratings disposed in the optical fiber, wherein the plurality oftemperature fiber Bragg gratings are interspersed between the pluralityof overheat fiber Bragg gratings.
 7. The system of claim 6, wherein theplurality of temperature fiber Bragg gratings are further configured toindicate a second calibration value, wherein the second calibrationvalue is based upon center wavelengths of the plurality of temperaturefiber Bragg gratings.
 8. A method of calibrating a fiber optic system,the method comprising: emitting, by a first optical transmitter disposedin a first interrogator connected to an optical fiber, a first opticalsignal into the optical fiber, wherein the optical fiber comprises: aplurality of overheat fiber Bragg gratings disposed in the opticalfiber; and a plurality of calibration fiber Bragg gratings disposed inthe optical fiber; reflecting the first optical signal with at least oneof the plurality of calibration fiber Bragg gratings to create a firstresponse signal; receiving, by a first optical receiver in the firstinterrogator, the first response signal from the optical fiber basedupon the reflected first optical signal; detecting the received firstresponse signal to identify presences of each of the plurality ofcalibration fiber Bragg gratings; emitting, by a second opticaltransmitter disposed in a second interrogator that is different from thefirst interrogator, a second optical signal into the optical fiber;reflecting the second optical signal with at least one of the pluralityof calibration fiber Bragg gratings to create a second response signal;receiving, by a second optical receiver in the second interrogator, asecond response signal from the optical fiber based upon the secondoptical signal; detecting the received second response signal to furtheridentify presences of each of the plurality of calibration fiber Bragggratings; and determining a calibration value based upon the identifiedpresences of the plurality of calibration fiber Bragg gratings.
 9. Themethod of claim 8, wherein the system further comprises a plurality oftemperature fiber Bragg gratings disposed in the optical fiber, whereinthe plurality of temperature fiber Bragg gratings are interspersedbetween the plurality of overheat fiber Bragg gratings.
 10. The methodof claim 9, further comprising: identifying a center wavelength of atleast one of the plurality of overheat fiber Bragg gratings and theplurality of temperature fiber Bragg gratings based upon the detectedpresences of the plurality of calibration fiber Bragg gratings.
 11. Themethod of claim 8, wherein determining a calibration value furthercomprises: assigning a calibration value to a line replaceable unitbased upon the detected presences of the plurality of calibration fiberBragg gratings; and communicating the calibration value of the linereplaceable unit to a controller operatively connected to the opticalreceiver of the first interrogator.
 12. The method of claim 8, furthercomprising identifying calibration values for all of the fiber Bragggratings of a line replaceable unit.
 13. The method of claim 8, furthercomprising determining a distance from the first interrogator to atleast one of the plurality of calibration fiber Bragg gratings basedupon the calibration value.
 14. The method of claim 8, furthercomprising identifying a center wavelength for each of the fiber Bragggratings based upon the calibration value.
 15. A detection systemcomprising: a line replaceable unit comprising: first and secondconnectors in optical communication; and an optical fiber extendingbetween the first and second connectors, the optical fiber with firstand second ends, wherein the first end of the optical fiber is connectedto the first connector, wherein the optical fiber comprises: a pluralityof overheat fiber Bragg gratings disposed in the optical fiber; a firsttiming fiber Bragg grating disposed in the optical fiber, wherein thefirst timing fiber Bragg grating is configured to indicate at least oneof a start point and end point of the line replaceable unit; a pluralityof calibration fiber Bragg gratings located in a pattern that providesinformation related to a calibration value of the line replaceable unitbased upon a center wavelength of each of the first plurality ofoverheat fiber Bragg gratings; a first interrogator connected to theline replaceable unit at the first end of the optical fiber, wherein thefirst interrogator is configured to provide a first optical signal tothe optical fiber and to receive a first optical response signal fromthe optical fiber; a second interrogator different from the firstinterrogator, wherein the second interrogator is connected to the secondend of the same optical fiber that the first interrogator is connectedto, wherein the second interrogator is configured to provide a secondoptical signal to the optical fiber and to receive a second opticalresponse signal from the optical fiber; and a controller operativelyconnected to the first interrogator and configured to determine thecalibration value of the line replaceable unit based on the firstoptical response signal.
 16. The detection system of claim 15, whereinthe overheat detection system is configured to be installed in anaircraft.
 17. The detection system of claim 15, wherein the controlleris configured to control the optical transmitter and determine the atleast one temperature for each of the plurality of zones using at leastone of time division multiplexing (TDM) and wavelength divisionmultiplexing (WDM).
 18. The detection system of claim 15, wherein thefirst interrogator further comprises: an optical transmitter configuredto provide the optical signal to the optical fiber; a first detectorconfigured to receive a response signal from the optical fiber; and acoupler connected to the optical transmitter and to the detector.